If the poster is canvas backed, you might be able to take it to a picture framing place, and get the tear(s) mended at the back with light canvas or paper tape. A canvas backed map is worth the bother. You might even think about getting the mended map laminated to protect and preserve it. The line will be still be visible from the front but the structural integrity will be nearly as good as new. Basically, they’ll flatten the map and then tape it from behind. You could do this yourself on a large tabletop I think - press it flat with a cool iron and then stick it together on the back. Please don’t use duct tape - that could ruin it.